A plastic lens is light in weight, has excellent toughness, and can easily be dyed. Properties that are particularly required for a plastic lens include physical and chemical properties such as low density, high transparency, reduced yellowness, high heat resistance and high strength, and optical properties such as high refractive index and high Abbe's number. A high refractive index allows thinning of the lens while a high Abbe's number reduces chromatic aberration of the lens.
Recently, a number of examples using an organic compound having a sulfur atom have been reported for the purpose of achieving high refractive index and high Abbe's number. Among them, a polyepisulfide compound having a sulfur atom is known to have good balance between the refractive index and the Abbe's number (Patent document 1). Moreover, since polyepisulfide compounds can react with various compounds, their compositions with various compounds have been proposed for enhancing physical properties (Patent documents 2-5).
Since, however, polyepisulfide compounds are hard to store for a long time due to its high reactivity, a technique of storing by refrigeration (Patent document 6) and a technique of adding an epoxy compound having a halogen group (Patent document 7) have been proposed.
In addition, coloring, striae or turbidity may be caused in a polymerized resin upon producing an optical material resin. There is a report that an optical resin with no coloring and no striae can be produced by keeping the content of the nitrogen component in a polythiol compound to lie in a specific range in order to enhance the yield (Patent document 8).